Not sure when the timing belt interval was changed but it is retrospective, all IS200 should have timing belts changed at 60,000 miles. At the very least it should be inspected at 60,000 miles and every 10,000 miles afterwards. Does your timing belt stamp indicate it was performed by a Lexus dealer? If so ring them up and see if they have a record of the work having been carried out.
Lexus Edinburgh are very good but I understand your desire to find a less expensive alternative.